Re: [Xen-devel] [PATCH net-next] xen-netback: fix xenvif_count_skb_slots()

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: David Vrabel
> Sent: 07 October 2013 10:50
> To: Paul Durrant
> Cc: xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Wei Liu; Ian Campbell;
> Annie Li; Matt Wilson; Xi Xiong
> Subject: Re: [Xen-devel] [PATCH net-next] xen-netback: fix
> xenvif_count_skb_slots()
> 
> On 04/10/13 17:26, Paul Durrant wrote:
> > Commit 4f0581d25827d5e864bcf07b05d73d0d12a20a5c introduced an error
> into
> > xenvif_count_skb_slots() for skbs with a linear area spanning a page
> > boundary. The alignment of skb->data needs to be taken into account, not
> > just the head length. This patch fixes the issue by dry-running the code
> > from xenvif_gop_skb() (and adjusting the comment above the function to
> note
> > that).
> 
> If 4f0581d2582 is causing the skb->data to be fully packed into a
> minimal number of slots then the simple
> DIV_ROUND_UP(skb_headlen(skb))
> is correct.
> 
> I think this change will miscount in the number of slots,
> over-estimating the count which I think will eventually cause netback to
> think the ring has no space when it has some.
> 
> Is the problem here not the miscounting of slots but running out of
> space in the grant table op array because we know use more copy ops?
> 

Essentially yes. Netback is built on the assumption of no more than two grant 
copies per ring slot.

> I didn't think there was any real merit in the problematic commit (or at
> least there was no evidence that it was better) so I would suggest just
> reverting it instead of trying to fix it up.
> 

I'd be happy with a reversion.

> If we do want to change how netback fills the ring then netback needs
> some redesign (i.e., change it so it doesn't have to this counting in
> advance) to make it much less fragile to changes in this area.
> 

Yes, that would be much better.

  Paul

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